Fortify Magicka effects can be used to increase your character's available magicka above the base value.

Your magicka will go down after casting a spell. It is restored over time at a rate determined by your willpower unless you are afflicted by Stunted Magicka, in which case your magicka will not naturally recover. The two most common ways to acquire Stunted Magicka are either by selecting the Atronach birth sign or by catching a disease with this effect (in particular Astral Vapors, which can be caught from Dread Zombies). You can also wield the Fork of Horripilation from the Shivering Isles Expansion Pack, which gives you stunted magicka. Tips on how to restore magicka when affected by Stunted Magicka are provided on the Atronach page.

Your magicka determines the strongest spell that you can cast. However, the magicka cost of a spell is also determined by your magical skills (see Magic Overview), so spells that at first appear impossibly expensive can become more affordable later in the game.

Waiting until your Intelligence is at 100 to do the Hermaeus Mora Daedric quest, allows you to get your base Intelligence to 110.

All combined, these would yield a total Magicka of 1032, which is the highest constant effect amount possible in the game without any mods. Higher values are temporarily possible by also using potions, Doomstone powers, scrolls, and/or spells.